Exposure of workers in the material processing industry to excessive noise is of primary consideration to plant operators, particularly because of MESA regulations. For example, a room in a coal preparation plant may contain six vibrating screens and two pumps and have a sound level of 98 dBA, which is well above the MESA-permitted 90 dBA, 8-hour limit. As a consequence the plant operator must either purchase equipment which results in a sound level no more than 90 dBA or purchase standard equipment having a higher noise level such as 98 dBA and modify worker routines to limit the exposure time of the worker to the higher noise level. Accordingly it is an object of this invention to provide a quieter vibrating screen, the noise level of which is reduced in comparison to prior art devices.
It is well known to freely support a vibratory screen body from a stationary support by resilient rubber shear mounts such as disclosed in U.S. Pats. Nos. 2,212,550; 2,729,332; 3,029,947 and 3,030,098 to permit a large amplitude of vibration in the desired direction while vibrationally isolating the screen body from the stationary support so that it will vibrate essentially as a free mass system, but such resilient mounting does not solve the problem of high levels of radiated noise from the vibrating screen.
It is also taught by such patents as U.S. Pat. No. 2,353,492 and U.S. Pat. No. 3,468,418 to resiliently support a rotatable eccentric mass drive mechanism on a freely floating screen body through rubber shear springs whose natural vibratory frequency, or spring rate approximates the operating frequency of the drive mechanism for the purpose of increasing the amplitude of vibration, or throw of the screen body. Further, such patents as German Pat. No. 975,330 disclose a vibrating screen device wherein a counterbalance mass is resiliently connected to the freely floating screen body through rubber shear mounts whose natural vibratory frequency is approximately equal to the operating frequency of a rotatable eccentric mass drive for the purpose of increasing the throw of the screen body. Still further, it is known from such patents as U.S. Pat. No. 3,089,582 to mount a rotatable eccentric mass drive mechanism on a freely floating screen body through rubber shear spring mounts to define a resonant spring-weight system wherein the vibratory forces transverse to the desired direction are absorbed in torsion by the shear springs whereas forces in the desired direction to move the screen body are transmitted in shear by the rubber mounts to thereby increase the throw of the screen body. However, none of such resilient mounting systems result in a vibrating screen having a substantially lower sound level.
It is a specific object of the invention to provide a vibrating screen with means for isolating harmonics of the shaking frequency from the screen body while transmitting the vibratory forces from the drive mechanism at the fundamental shaker frequency unrestrained to the screen body, thereby minimizing noise which would otherwise radiate from the side wall and end wall panels of the screen body as a result of such panels being excited at the harmonic frequencies. Particularly it is an object of the invention to provide a vibrating screen with a constant frequency rotatable eccentric mass shaker drive and having means for resiliently mounting the shaker drive on the screen body which transmits vibratory forces at the constant shaker frequency unrestrained to the screen body and substantially attenuates vibratory forces at frequencies which are harmonics of the shaker frequency and would, in the absence of such resilient mounting means, radiate noise by exciting the wall panels of the screen body.
A vibrating screen in accordance with the invention has a freely floating screen body for moving particulate material, rotatable eccentric mass drive means for vibrating the screen body at a substantially constant shaker frequency which is substantially higher than the natural vibration frequency of the screen body, and means for resiliently mounting the drive means on the screen body so that vibratory forces emanating from the drive means at approximately the shaker frequency are transmitted unrestrained to the screen body and vibratory forces above the shaker frequency are substantially attenuated, said resilient mounting means constituting substantially the sole connection between the drive means and the screen body and the natural vibrating frequency of the resilient mounting means being approximately equal to the constant shaker frequency divided by 2.